doc: add CONTRIBUTING.md
doc: add CONTRIBUTING.md

Change-Id: I1daab4196856b99cd60db57e917f8c6f07f3eefa
Reviewed-on: https://go-review.googlesource.com/5412
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

AkihiroSuda/golang-exp-aspectgo,golang/exp,golang/exp,AkihiroSuda/golang-exp-aspectgo,AkihiroSuda/golang-exp-aspectgo,golang/exp,go-src/inotify,JamesDunne/exp,JamesDunne/exp